In the Eye of the Beholder

604 Br J Ophthalmol 2002;86:604 Br J Ophthalmol: first published as 10.1136/bjo.86.6.604 on 1 June 2002. Downloaded from Coverillustration...... In the eye of the beholder

f fees were charged for the sight of a as being around the sun rather than to include the angular momentum of rainbow, Cambridge University should being in the antisolar horizon, as are the photons, taking the explanation of the Ireceive a commission. Our under- first and second order bows. rainbow to yet another mathematical standing of the rainbow’s mysteries level beyond the scope of this essay. came principally from the faculty of But this does not give us the rainbow. Cambridge, including professors Isaac Instead, it provides us with energy waves Newton, Thomas Young, George Airy, of a certain wavelength at a certain Richard Potter, and Lord Rayleigh. Each angle, but not the rainbow. The beautiful of these men provided important steps colours that create the rainbow are only towards our current understanding of “seen” by the brain although excitation the family of bows, which includes the occurs at the photoreceptors. rainbow as its most famous member. Part of our understanding of percep- Early references to the rainbow are tion comes from Thomas Young who, in a made in the Bible where it is written that Bakerian Lecture, correctly predicted the Noah was given the rainbow as a gift and trichromacy theory of vision and three a covenant that the earth would never photoreceptors as the tools to achieve again be destroyed by water. Homer, in colour perception. the Iliad, describes Iris as the goddess of In the early 1800s, Thomas Young, Helmholtz and Maxwell further ex- the rainbow. Iris was regarded as a mes- educated at Cambridge, explained the panded the theory of colour vision, but senger, or bridge, between the gods, supernumerary bows that can frequently did not discuss perception. In fact, the especially when they intended discord. be seen within the primary rainbow. He human eye/brain can distinguish 8000 The Greeks began the scientific inves- championed the wave theory of light to colours at a single luminance and a total tigation into the rainbow, and claim sev- explain these interference bows al- of eight million shades and tints. Visual eral important advances. Alexander of though this was at odds with the processing does all this and the brain Aphrodisias was the first to document corpuscular theory of light put forth by must make many assumptions. As a the dark strip between the two arcs of the revered Newton, and it took others result we are blessed with colour con- the double bow (visible on the cover), such as Fresnel and Richard Potter stancy, that feature which allows us to and the phenomenon bears his name (again of Cambridge) to convince the perceive a lemon as a lemon in almost today as Alexander’s phenomenon. Aris- world of physics that Young was correct. any light level. We still see a lemon in totle, about AD 322, wrote that tiny George Airy, professor of astronomy at different light even though the photo- http://bjo.bmj.com/ mirrors in the clouds were responsible Cambridge, further explained the inter- metric measurement of the wavelength for the reflected light that created the ference bows, the mathematics of dif- given off by the lemon in bright noon- bow. He stated that the colours were fraction, and took the explanation of the time sun is vastly different from the caused by a weakening of the light. His family of bows to a new mathematical wavelength of light given off by the same explanation, which held sway for 1500 level when he stated that light passing lemon in twilight. Yet, we don’t know years, was wrong. through the raindrop was a caustic. A how the brain does this. Other men began to understand and caustic is a curved wavefront such as one The 17th century with Descartes, on September 25, 2021 by guest. Protected copyright. come closer to the correct explanation would see when light is reflected from Newton, and others, and the 19th cen- but were generally ignored until the 17th the surface of a smooth, curved silver tury with Young, Airy, Maxwell, Helm- century. At that time, Rene Descartes, ring. Airy further extended the under- holtz, Hertz, and others were important with help from Harriot, Snell, Kepler, standing of the wave theory of light and centuries in the understanding of the and others, correctly described the two thought the explanation of the rainbow rainbow. Perhaps the 21st century is the refractions and one reflection from each complete. time for another major breakthrough in droplet that created the geometry of the But the 38° bow, often known as a fog the true understanding of the family of first and second bow, but he could not bow, eluded Airy’s explanation. It re- bows, and now the horizon is perception. explain the colours. mained for Lord Rayleigh to alter Airy’s Newton, born at Woolsthorpe Manor, theory to explain bows from extremely I R Schwab, illustrated on this month’s cover, cor- small droplets of water. James Maxwell, University of California, Davis, Department of rectly described the refraction of light of colour wheel fame, would later pro- Ophthalmology, 4860 “Y” Street, Suite 2400, that would create the colours of the bow. pose that light was a form of mechanical Sacramento, CA 95817, USA; Newton was also most instrumental in vibration providing the basis for the [email protected] the understanding of the rainbow with electromagnetic wave theory and thus The sensational cover photograph of a double his creation of the calculus as a branch of unifying existing concepts. rainbow over Woolsthorpe Manor, Sir Isaac mathematics. With Newton’s calculus, Others, including Poincare, Watson Newton’s birthplace, was taken by Roy L Sir Edmund Halley (of comet fame) cal- and, later, Feynman, would devise the Bishop of Nova Scotia. He graciously con- culated the third and fourth order bows quantum electrodynamic theory of light sented to our publication of this photograph.